1. Field of Invention
This invention relates to articles having sliding surfaces, and more particularly relates to a lubricating system effective across a maximum range of velocities useful for articles having sliding surfaces.
2. Background
Certain devices require slow and controlled initiation and maintenance of sliding movement of one surface over another surface. It is well known that two stationary surfaces having a sliding relationship often exhibit sufficient resistance to initiation of movement that gradually increased force applied to one of the surfaces does not cause movement until a threshold force is reached at which point a sudden sliding separation of the surfaces takes place. This sudden separation of stationary surfaces into a sliding relationship is generally referred to as breakout, and the stationary period is generally referred to as stick.
Often, immediately after breakout, sliding movement stops and the two surfaces once again go into stick, only to undergo another breakout with time and continued force. The condition of alternating stick and breakout is generally referred to as chatter.
Breakout and chatter are particularly troublesome in liquid dispensing devices using stopcocks, such as burets, pipers, addition funnels and the like where careful dropwise control of flow is desired. The problem of breakout and chatter is most severe in devices, such as syringes, used to deliver small, accurately measured quantities of a liquid by smooth, incremental line-to-line advancement of one surface over a graduated second surface.
Syringes are commonly used either manually, i.e., when stopper advancement is achieved by holding the syringe in the hand and applying finger pressure on the barrel, or automatically, i.e. in conjunction with an infusion pump. Manually operated syringes develop a stopper velocity of from about 20 to 1000 mm per min., and usually are operated at hundreds of mm per min. In the present disclosure, this velocity is referred to as manual velocity.
On the other hand, syringes used with an infusion pump are designed to deliver a medicament over a long period of time. In these devices the syringe stopper travels at ultra slow velocities, from about 0.003 to about 2.0 mm/min. This stopper velocity is hereinafter referred to as the pump velocity.
It will be readily apparent that chatter is most likely to develop when the surfaces must move very slowly relative to each other, i.e., at pump velocity. Further, these velocities cause frictional forces to increase substantially, so that the barrel and stopper are often in stick, during which time no medicament is dispensed for extended periods. When breakout does occur, a bolus may be administered. Subsequent stick followed by breakout leads to chatter.
U.S. Pat. No. 4,767,414 to Williams et al. discloses a method for overcoming breakout for a hand-held syringe. The method includes applying a silicone lubricant to a surface slidably engaged with a second surface and treating both the surface and the silicone with an ionizing plasma.
Karakelle et al., in U.S. Pat. No. 4,844,986 discloses plasma polymerization of a silicon containing monomer and deposition of the polymer on a polypropylene surface so that both the surface and the polymer are crosslinked. This surface is taught to be non-lubricious, but has a surface energy which allows it to be coated evenly with an ordinary polydimethylsiloxane lubricant.
Since off-the-shelf syringes are used both manually and with a pump, there is a need for a lubricant system effective at both manual and pump velocities. This need is fulfilled by the present invention.